In the past, methods for achieving smaller feature sizes have been to select a lithographic radiation source having a shorter wavelength, increase the numerical aperture (NA) of the lithographic system's lens or a combination thereof. While these methods have met with success, for each reduction in wavelength and/or increase in NA, the problems associated with taking advantage of such changes have been increasingly difficult to overcome.
Recently it has been suggested that rather than selecting a new lithographic radiation source with a shorter wavelength, e.g. 157 nm, the resolution of the current 193 nm standard source could be extended by the use of an immersion lithographic process. Such immersion lithographic processes replace the usual “air gap” between a lithographic tool's final lens and the substrate being exposed with a fluid such as, for example, water. The water, having a refractive index that is much greater than that of air, allows for the use of a lens having a NA higher than 1 without the reduction in depth of focus (DOF) that would otherwise result. Thus, it is believed that minimum feature sizes of 45 nm or less can be achieved with such an approach.
However, the successful implementation of immersion lithography for microelectronic device fabrication presents new problems that need to be resolved. For example, typically the substrate being exposed during a microlithographic process is repeatedly repositioned with respect to the lithographic tools lens to achieve complete exposure of all portions of the substrate. The presence of an immersion fluid (also referred to herein as an “immersion medium” or “IM”) raises the concern that fluid residues will result from this movement and that such residues will result in an increase in defectivity that would make such a process unacceptable. Also with regard to fluid residue, or residuals, it must be considered that any proposed solution to this problem should not result in a significant decrease in the speed with which such movement is currently accomplished, as such a decrease in movement speed (scanability) could result in an unacceptable decrease in the number of substrates per hour that a lithographic tool can fully expose.
In addition to problems relating to IM residuals and scanability, the use of an IM also raises concerns with regard to problems that can result from such a fluid being in direct contact with the photoresist layer that can lead to a reduction in that layers resolution ability. For example, such problems can include, among others: 1) leaching of small molecules such as photoacid generators (PAGs) and PAG photoproducts from the photoresist film into the IM and 2) absorption of the immersion medium, or components thereof, into the photoresist film.
One method that has been investigated for the elimination or reduction of these and other problems associated with immersion lithography is the use of an intervening layer disposed overlying the photoresist film. Such an intervening layer, also referred to as a “top-coat” or “protecting layer,” is thus positioned to receive the immersion material and thus prevent or greatly reduce any effects related to previously mentioned technical problems 1 and 2. With regard to scanability, the use of a top-coat allows for the design of a material having specific properties that will eliminate or greatly reduce the possibility of IM residuals with little or no reduction in the speed of a tool's speed of movement.
Recently, some materials encompassing fluorine-containing polymer(s) have been proposed for use as a top-coat layer. While such materials have been shown to have a positive effect with regards to the problems discussed above, they require the use of a solvent for their removal. As any top-coat layer must be removed to allow for the development of an image in the underlying photoresist layer, a material that requires that a special solvent be used for its removal is problematic in that such removal is an extra step that adds undesirable equipment and material costs as well as costs associated with the reduced productivity such an extra step will necessarily cause.
Therefore, it would be desirable to provide solutions that can be readily implemented, such solutions directed to the above-related technical problems that may occur with immersion lithography. Such solutions should provide for the reduction or prevention of the leaching of small molecules from a photoresist layer into an immersion medium as well as reduce or prevent the absorption of such immersion medium into such a layer. Such solutions should also serve to reduce defectivity from a level observed when immersion lithography is preformed without such solutions being employed. Further, it would be desirable for such solutions to be cost effective and not require significant alternative process such as observed with the aforementioned solvent removable top-coat material or any significant reduction in scanability when employed.